A conventional air passage opening/closing device is provided with a guide groove in a case that forms an air passage. The guide groove serves to guide both end portions in the door width direction of a slide door. With this arrangement, the slide door is designed to reciprocate along the guide groove. Such a movement of the slide door opens and closes the air passage of the case.
The radius of curvature of the slide door in a single non-mounted state is set equal to or more than that of the guide groove, so that both end portions of the slide door in the door movement direction are easily brought into contact with a seal surface on a case side when assembling the slide door in the case. Thus, even under a small volume of air, the slide door is pressure-contacted with the seal surface on the case side by pressure of blown air, so as to exhibit good sealability with the slide door.
However, while the air pressure is not applied to the slide door, the slide door is not pressure-contacted with the seal surface on the case side. When the entire case vibrates in this state, both end portions in the door movement direction of the slide door might vibrate, causing abnormal sound (noise) from the slide door.
An air passage opening/closing device disclosed in Patent Document 1 is provided with a guide groove composed of a windward sidewall portion and a leeward sidewall portion. The sidewall portions are respectively provided on both sides of the slide door in the door width direction. The radius of curvature of the slide door in a single non-mounted state is set equal to or more than that of each of the windward sidewall portion and the leeward sidewall portion. As a result, the center portion of the slide door in the door movement direction is pushed against the windward sidewall portion by an elastic deformation force of the door, while both end portions of the slide door in the door movement direction are pushed against the leeward sidewall by the elastic deformation force of the door. In this way, the slide door is supported at three points, including the center portion and both the end portions of the door in the door movement direction, thereby suppressing the vibration of both end portions of the slide door in the door movement direction.